The present invention generally relates to semiconductor memory devices, and more particularly to a semiconductor memory device comprising as a memory active element a resonant-tunneling hot electron transistor (hereinafter simply referred to as an RHET) which uses the resonant-tunneling effect.
Various kinds of semiconductor memory devices have been proposed, but there are still demands for semiconductor memory devices with even higher speed and higher integration density. However, due to the technical limit to the integration and the increase in the media delay time caused by the increased wire capacitance, it is reaching a stage of deadlock where the demands have been satisfied to a point near the limit according to the present technology of producing the semiconductor memory devices. In order to break this deadlock, the active element itself must operate at a higher speed and also have new functions, so that the number of active elements can be reduced and the higher speed can be achieved while maintaining the functions of the semiconductor memory device.
Presently, in order to produce a practical static memory cell, at least four transistors are normally required when field effect transistors are used. It is possible to produce a practical static memory cell by use of two transistors when bipolar transistors are used, but in this case, the bipolar transistors must have four terminals, that is, have a double emitter configuration.
In a new generation of further improved semiconductor memory devices, it is desirable that the semiconductor memory device not only operate at a high speed but also use a reduced number of active elements. But no such semiconductor memory device has yet been proposed, mainly because there is no existing memory active element suitable for use in such a new generation of semiconductor memory device for carrying out the desired functions.
Recently, the RHET which uses the resonant-tunneling effect has been proposed. The RHET is disclosed in a paper "A Resonant-tunneling Hot Electron Transistor (RHET)" by Naoki Yokoyama et al., the Proceedings of the Twelfth International Symposium on Gallium Arsenide and Related Compounds held in Karuizawa, Japan, Sept. 23-26, 1985 (Institute of Physics Conference Series No. 79, Chapter 13, Adam Hilger Ltd., Bristol and Boston), and also in an article "A New Functional, Resonant-tunneling Hot Electron Transistor (RHET)" by Naoki Yokoyama et al., Japanese Journal of Applied Physics, Volume 24, No. 11, November, 1985, pp.L853-L854. A flip-flop circuit using this RHET was previously proposed in a U.S. patent application Ser. No. 918,300 filed Oct. 10, 1986 in which the present inventor is one of the inventors. The present invention effectively uses this RHET to realize a semiconductor memory device having the desired functions described before.